In a magnified image projector apparatus such as a microfilm strip reader, printer or combination reader/printer apparatus, the image to be reproduced on the viewing screen is correctly focussed with the projection lens moved with respect to the viewing screen through manipulative efforts of the operator, or viewer, who drives the lens directly or handles the mechanical lens position control means provided in the apparatus. A magnified image projector apparatus having such a manually operated focus control arrangement is now being superseded by a microfilm strip reader, printer or combination reader/printer apparatus equipped with an automatic focus control system. In the magnified image projector apparatus having such an automatic focus control system, the location of the plane on which a beam of light is focussed by the projection lens is detected by means of a photodetector unit which is responsive directly to the magnified image reproduced on the viewing screen of the apparatus.
A problem is encountered in a magnified image projector apparatus having such a known automatic focus control system in that the photodetector unit to be used is required to have a disproportionately spacious light receiving area in order that the photodetector unit be capable of coping with the blank areas which frequently appear in the image frames of a microfilm strip or the margin areas between the successive image frames of the microfilm strip. When the image reproduced on the viewing screen of the apparatus has a blank area which is wide compared to the effective light receiving area of the photodetector unit, it may happen that the photodetector unit can not detect or accurately detect the location of the plane o which the beam of light is focussed by the projection lens.
In a still or television camera having a reduced image projection system, there is provided a path of light to be used for the detection of the plane on which the beam of light is focussed, in addition to the path of light along which an information-carrying beam of light is to be directed through the reduced image projection system. The path of light to be used for the detection of the plane on which the beam of light is focussed may be created with an additional focusing lens located behind a focal plane of the object lens. The optical arrangement of this nature however has a drawback in that incorporation of such optical arrangement into the reduced image projection system results in an increase in the distance of the path of light from the projection lens to the photodetector unit. Another problem using the optical arrangement is that not only the scale but also the movement of an original image are magnified in the magnified image projection system so that the image to be focussed by the projection lens might enter the focal plane of the additional focussing lens and could not produce an reproduced image o the photodetector unit.
The path of light to be used for the detection of the location at which the beam of light is focussed may be created with a half mirror disposed between the front and rear lens groups of the object lens system. An additional focusing lens is located in one of the two components into which the beam of light to be photographed is split by the half mirror. This type of optical arrangement makes it difficult to exchange the lens system with another one.